Sensors are known that measure environmental conditions such as temperature, pressure, and relative humidity. For example, thermometers measure temperature and barometers measure pressure. One type of moisture sensor is called a wet/dry bulb thermometer. Typically, such devices includes two thermometers, with one thermometer having its bulb wrapped in a material that is wetted with water. As the water evaporates from the wet thermometer, it absorbs energy from the bulb causing the temperature to drop. By comparing the temperature of the wetted thermometer with that of the normal thermometer, one can determine the relative humidity of the air, with the understanding that more humidity in the air results in a slower evaporation rate and a smaller difference in temperature between the two thermometers.
Other types of relative humidity sensors include moisture sensitive materials that may change color depending upon the moisture level, and capacitors that include water permeable dielectrics. In the latter, the capacitor is part of a circuit, and depending on the humidity, which senses the capacitance changes. This change in capacitance can be related to the relative humidity.
There are a number of problems with the above humidity sensors. For example, the wet/dry bulb thermometer is relatively bulky and, because it has a relatively heavy mass, it takes a significant amount of time to register the changes in temperature that are related to the changes in humidity. The chemical detectors often may only be used once, and may be slower to register a reading as well.
With respect to the capacitor device mentioned above, its use may be limited by its size. More specifically, such devices must have a relatively large capacitor so that changes in the dielectric constant, and thus the capacitance, are large enough to be read by attendant circuitry.
A further problem with all of the humidity sensors mentioned above is that they are not readily useable in remote locations. This is due to the fact that additional circuitry and apparatus is needed to convert their readings into a signal that may be transmitted to a remote receiving station. Similar problems plague other environmental sensors such as temperature and pressure sensors.